The purpose of this study is to determine the absolute and relative bioavailability of valacyclovir compared to intravenous and oral acyclovir respectively. Six liver and six renal transplant recipients will participate in this study. Patients will receive a single oral 500 mg dose of valacyclovir, a single oral 800 mg dose of acyclovir and a single 200 mg intravenous dose of acyclovir on three separate occasions. The pharmacokinetics of acyclovir and valacyclovir will be characterized. This study will provide information on equivalent dosing.